PURPOSE: To obtain a coupling structure to an optical fiber of an optical waveguide, which has a sufficient coupling degree and a simple structure, and is small-sized by constituting so that a width and a thickness of a coupling part to an optical fiber of a core part of an optical coupling waveguide are about the same as a diameter of a core of the optical fiber, and also the center axis of the core part in this coupling part is inclined against the center axis of an optical waveguide of a substrate.

CONSTITUTION: As for an optical coupling waveguide 10, a core part 13 is provided between a clad layer 11 and 12 formed on one end of a substrate 1. An end face of a coupling side of the clad layers 11, 12 and an optical fiber 3 of the core part 13 forms a command face with one end face of a substrate 1, and the other end of the core part 13 is coupled with an optical waveguide 2. A width and a thickness in the coupling part with the optical fiber 3 of this core part 13 is made roughly equal to or larger than a diameter of a core 4 of the optical fiber 3. Also, the clay layer 11 is thinned gradually as it is separated from the coupling part with the optical fiber 3. Accordingly, in the coupling part of the core part 13 and the optical waveguide 2, the center axis X of the core part 13 is inclined against the center axis Y of the optical waveguide 2. In this way, the greater part of an energy of the light from the optical fiber 3 is coupled to the optical waveguide 2 of the substrate 1.